The present invention relates generally to bonded wafers and more specifically to an improved method of wafer bonding.
Wafer bonding is a technique widely used as a processed step in fabricating SOI (silicon on insulator) and DI (dielectric isolation) in an integrated circuit structure. The starting material is on one slice and is to be bonded to a handle on a second slice. At least one if not both of the bonding surfaces of the two slices has thermally grown oxide thereon. The two prepared slices are put together in the presence of air and heated at a certain temperature, generally above 1000.degree., to accomplish bonding. This may be performed in an oxidizing atmosphere if preferred.
During further processing of the wafer by thinning one of the two slices, generally the non-handle slice, delamination occurs over part of the bonded surface. This delamination seems to be at least partially due to pitting over the bonding surface.
The prior art wafer bonding process has a high yield rate if the measure is only bonding of the wafers. If the measure is the percentage of surface area bonded, the yield is drastically reduced. The prior art 100% bonded surfaces have a yield of about 20%.
Thus it is an object of the present invention to provide a bonding process which removes pitting of the bonding surfaces.
Another object of the present invention is to eliminate nitrogen from the bonding surfaces prior to bonding so as to minimize pitting.
An even further object of the present invention is to maximize the formation of a vacuum and facilitate the compliance of the two bonding surfaces during bonding.
A still further object of the present invention is to increase the yield of 100% bonded wafers.
These and other objects of the present invention are attained by pressing a pair of slices together with a liquid oxidant there between and subjecting the pair of slices to a temperature to bond the slices together. Preferably a liquid oxidant is applied to one of the slices before they are pressed together to-form the substantially nitrogen free oxidant between the pressed slices. Preferably the oxidant is water or water in combination with hydrogen peroxide. The pair of slices with the oxidant there between is dried generally at room temperature in a pure oxygen or air atmosphere prior to the heating step which produces bonding. The bonding surfaces of the slices are treated with an acid to produce hydroxyl groups before applying the oxidant. Preferably only one of the bonding surfaces is oxidized prior to being pressed together. The heating step for bonding is carried out at a sufficiently high temperature of at least 1100.degree. C. to make the slices pliable so as to comply with each other during the bonding step. By providing a liquid oxidant, oxidation is maximized and the vacuum produced by the depletion of the gaseous oxidant in combination with the pliable substrates produces the maximum conformance of the two slices. This improves the bonding process and protects against delamination.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.